1. Field of the Invention
The present invention relates to a wavelength-division multiplexed optical transmission system, more particularly, to a wavelength-division multiplexed optical transmission system which is capable of reducing inter-channel correlation that may cause a serious transmission quality degradation.
2. Description of the Related Art
In these days, according to digital standardized information communication used for an optical fiber transmission system, there is widely utilized a frame synchronous communication system which is called an SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy)
According to the SONET/SDH standard, relative phase difference information between a data signal and a frame phase, what is called a pointer, is given to an overhead area in a frame and transmitted, thereby enabling the change in phase difference between the data signal and the frame phase.
Since signals transmitted from a plurality of different stations incur transmitting delay and jitter/wander, the frame phases are made various. Then, a receiving station exchanges a pointer so as to match the transmitted frame phases with a frame phase of the station reference. The all of frame phases based on the SONET/SDH standard are as same as the frame phase based on the station reference after the pointer exchanging process and therefore signal processes such as exchange can be executed easily.
According to the SONET/SDH standard, a scrambling/descrambling method for suppressing the sequence of the same symbol (such as 0,0,0,0 . . . or 1,1,1,1 . . . ) is also defined. Specifically speaking, it is defined to use a binary NRZ (Non-Return to Zero) format which is scrambled and to set to be equal to (1+X6+X7), a polynomial for generating a pseudo random (PRBS: Pseudo Random Bit Sequence) pattern which is used for scrambling. FIGS. 12 and 13 show a functional block of scrambling/descrambling and a constructional example of a PRBS pattern generator.
That is, referring to FIG. 12, based on the PRBS pattern which is generated by a PRBS pattern generator 131a, input data is scrambled by a gate circuit 131b on a transmitter 131 side, and the scrambled data is transmitted to a receiver 231 side via a transmitting line 500. Based upon the PRBS pattern generated by a PRBS pattern generator 231a, data scrambled by the gate circuit 231b is descrambled on a receiver 231 side.
Referring to FIG. 13, the PRBS pattern generator 131a comprises flip-flops 41 to 47 and a gate circuit 48, and generates the PRBS pattern on the basis of a bit rate clock and a frame pulse which are inputted and transmits the generated PRBS pattern to the gate circuit 131b. 
Incidentally, according to a recent optical transmission system, the introduction of a so-called wavelength multiplex/demultiplex technique is started, whereby a plurality of channels having different wavelengths are transmitted in a single optical fiber, so as to respond to the increase in transmitting capacity demand.
As for one merit of the wavelength multiplexing method, it is exemplified to handle wavelength channels as if they were transmitted in an individual fiber. Therefore, the wavelength multiplexing technique is introduced and the capacity is increased, without drastically changing the SONET/SDH standard which took no account of the wavelength multiplex. As a result, generally, a plurality of SONET/SDH channels are simultaneously transmitted in a single optical fiber as an operating form in these days.
However, a problem might arise, when wavelength-multiplexing the SONET/SDH signal which took no account of the wavelength multiplexing transmission as it is. In particular, according to a conventional SONET/SDH system, there is a problem to have no guarantee on no correlativity for data among a plurality of wavelength channels which are transmitted in parallel in the optical transmitting line. In the case where the data among the parallel transmitted channels is strongly correlative, it is dangerous to cause the serious deterioration in transmission. This dangerous point will be described hereinlater.
It is known to degrade a signal waveform due to a non-linear phase modulating effect which is caused in the optical fiber upon transmitting the data in the optical fiber. According to the non-linear phase modulating effect, the refractive index of optical fiber is varied by depending on optical intensity, thereby phase modulating the optical signal which is propagated in the optical fiber. The phase modulating component is converted into an intensity modulating component by GVD (Group Velocity Dispersion: chromatic dispersion) in the optical fiber, and to thereby become a waveform distortion.
It is considered that there are a non-linear phase modulation caused by intensity change of a noticed channel itself (SPM: Self Phase Modulation) and a non-linear phase modulation caused by intensity change of a channel travelling in parallel (XPM: Cross Phase Modulation), respectively. Particularly, the XPM is phase modulation caused by a signal having no correlation with information which the XPM itself has, and thus the transmitting characteristic is deteriorated certainly.
The next description turns to a case of causing an abnormally large XPM. FIG. 11 is a schematic diagram of a typical wavelength-division multiplexed optical transmission system. Referring to FIG. 11, a wavelength-division multiplexed optical transmission system comprises: a transmitting side including bit error rate measuring equipment 101, terminals 111 to 126, optical transmitters 131 to 146, and a multiplexer 150; and a receiving side including bit error rate measuring equipment 201, terminals 211 to 226, optical receivers 231 to 246, and a optical demultiplexer 250. Note that the optical transmitting line, which connects the transmitting side to the receiving side and comprises optical amplifiers 301 and 302 and optical fibers 401 and 402.
If normally operating the wavelength-division multiplexed optical transmission system, as shown in FIG. 14, data through which wavelength channels transmit has no correlation each other and data is never correlative. However, if all of the channels except for any noticed channel (referred to as λ3, herein) have the same data and same phase, as shown in FIG. 15, an abnormally large XPM occurs in the noticed channel. Although such a situation should be prevented as much as possible, there is a great of risk for the conventional apparatus to get into such a situation.
First, the frame phase matching is caused by matching the frame phase with the reference frame phase which is distributed from the station in a flow of the signal process. The data matching hardly occurs upon normal operation and, however, there is a possibility that the data matching is caused exceptionally, such as alarm transferring case.
A transmitter in the SONET/SDH system is defined to input dummy data into a data payload portion and continue to transmit the dummy data, even if no data to be communicated is inputted. This is the reason to continue to communicate the clock signal and communicate information in the overhead portion, which is stored into a portion other than the data payload.
There is a specific case of alarm transfer what is called an AIS-L based on the SONET/SDH standard. In this case, it is defined that the whole data payload portion is full of “1”, scrambled normally, and transmitted. The data included in the data payload portion becomes a scrambling pattern itself. Consequently, transmitting patterns among the channels are the same and the correlativity is extremely strong.
As another problem which arises by making the frame phase similar, there is a cross gain modulation effect (XGM: Cross Gain Modulation) in an optical amplifier in these days. There is typically used a laser amplifier using an optical fiber, to which an active element (rare earth element such as erbium) is doped, as an optical amplifier. Since the amplifier amplifies a wave so as to collect light which has been wavelength-multiplexed as single light, the intensity change in any desired channel can be transferred to the intensity fluctuation of another channel. That is called a cross gain modulation effect (XGM) and has a close relation to a relaxation oscillation frequency in the laser amplifying system.
The transfer function has a low pass filter characteristic and the relaxation oscillation frequency is a cut-off frequency of low-pass filter. Specifically speaking, the relaxation oscillation frequency is equal to 1 to 3 kHz, in case of using a Erbium-doped fiber amplifier (EDFA).
Corresponding thereto, there is a problem how low frequency component is included in the transmitted signal. The frame frequency period of the SONET/SDH signal is equal to 8 kHz, so that the 8 kHz-frequency component must be transmitted without a problem. Although this value is finely over a value of the cut-off frequency, it is understood that there is no allowance for transmission.
If the frame phases among the channels are ununiform upon amplifying the wavelength-multiplexed light, it can be expected that the frame frequency components are made uniform and are not proportional to the number of channels, when seeing the ununiform frame phases as a whole. On the contrary, if the frame phases are made uniform, it is remarkably dangerous to transfer the intensity fluctuation to a channel of a channel except for the noticed channel.
Actually, if making a channel excluding the noticed channel have the same data and the same frame phase in an examining system as shown in FIG. 11, a serious bit error rate degradation is observed in the noticed channel.